In order to combat the problems of multipath fading, distortion, and momentary loss of signal, automative FM receivers have typically been designed with some sort of audio processing in the form of blend, high-frequency rolloff, or audio attenuation. Typically, these audio processing functions are applied through the sensing of signal level (AGC) or high-frequency disturbances (ultra-sonic noise). An audio processor administers appropriate processing functions to L+R and L-R stereo decoder outputs and recombines the signal to produce Left and Right audio output.
Blend may be defined as the reduction of stereo separation produced by reducing the L-R signal. Blend is useful because in the process of decoding the composite audio signal from the FM detector, a much larger noise bandwidth is opened along with the decoded L-R signal. This inherent noise theoretically contributes 26 dB more noise when the receiver is in full stereo than when it is in the mono mode. Blend, therefore, can be called upon to attenuate this random, broadband noise during weak signal conditions.
In addition to the reduction of the broadband "stereo" noise, blend also reduces the noise and distortion created by multipath interference and co-channel and adjacent-channel interference. In these conditions, distortion and intermodulation (IM) products show up in great quantities in the L-R subbands. Consequently, when the Left and Right signals are decoded, these objectionable products are included in the program material, being perceived by the listener as noise.
The application of blend will, of course, result in the loss of stereo separation. In order to effectively reduce noise, this separation loss must be nearly total i.e. a substantial reduction of noise carries with it the entire loss of stereo separation. During reception of high stereo content signals such as instrumental music, a sudden loss of separation will be perceived as a collapse of the stereo image. A slighter, second-order effect is the apparent loss of some of the treble content of the music. Some of this loss is real i.e. elimination of the treble-rich L-R subband, but a large portion of the apparent loss is imagined. The imaginary component of the loss is due to reduction of noise. If the audio signal has little high-frequency information of its own, the reduction of noise sounds like a loss of treble. This is part of what is known as the "masking" phenomenon.
High-frequency rolloff deliberately limits the bandwidth of the audio signal to attenuate noise in the high-frequency portion of the audible spectrum. The implementation of this usually involves the dynamic control of a low-pass filter whose attenuation range is gradually moved into the high-frequency spectrum. Typically, maximum rolloff consists of a first-order attenuation curve (6 dB/octave) at a break frequency of 1.5-2 kHz. The rolloff function is applied to both L+R and L-R paths.
In order to further explain the nature of the audio processing functions, particularly rolloff, reference is made to FIGS. 1a and 1b. "Noise masking" refers to the ability of program material, with a given spectral density, to effectively "cover over" the presence of broadband (random) noise from the listener's viewpoint. This effect is strongly influenced by the ear's sensitivity to noise in different frequency ranges, as well as the triangular noise vs. frequency characteristic of an FM detector. FIGS. 1a and 1b illustrate noise masking for treble-rich music and a single tone respectively. Treble-rich music, has a very broad spectral density which tends to "mask" the presence of the noise. A single tone, on the other hand, tends to mask broadband noise only in the vicinity of the tone's characteristic frequency.
The extent to which rolloff may improve perceived S/N depends mainly upon the degree of "masking" of noise by the program material. The unavoidable consequence of rolloff is, of course, that any program content in the frequency range of rolloff action is attenuated along with the undesired noise. If the music masks the noise effectively, then application of rolloff does little or nothing to enhance the perceived S/N. It only serves to attenuate the program material high-frequency content in the ears of the listener. On the other hand, rolloff is very effective if little program material energy resides in the noise region i.e. both measured and perceived S/N improve dramatically.